The background art of the present invention will be described by taking a hydraulic excavator shown in FIG. 6 as an example.
The hydraulic excavator includes a crawler type of lower traveling body 1, an upper slewing body 2 mounted on the lower traveling body 1 so as to be slewed around an axis X perpendicular to the ground surface, and a working attachment 3 installed on the upper slewing body 2. The working attachment 3 has a boom 4, an arm 5, a bucket 6, and a plurality of hydraulic cylinders that drive these units, namely, a boom cylinder 7, an arm cylinder 8, and a bucket cylinder 9. The hydraulic excavator further includes a plurality of hydraulic motors which are hydraulic actuators other than the cylinders 7 to 9. The plurality of hydraulic motors include a traveling motor that drives the lower traveling body 1 and a slewing motor that drives the upper slewing body 2.
On the hydraulic excavator, mounted is an actuator circuit for driving each hydraulic actuator. The actuator circuit has a hydraulic pump, and a relief valve for limiting maximum pressure in the actuator circuit. The relief valve has a setting pressure (a relief pressure) defining maximum pressure of each hydraulic actuator. Specifically, the relief valve makes a relief action of returning a surplus component of hydraulic fluid discharged from the hydraulic pump to a tank to prevent the pressure of the hydraulic fluid in each hydraulic actuator from exceeding the relief pressure.
The relief action, however, involves a large pressure loss, namely, a relief loss, thereby degrading energy efficiency. For example, in the slewing circuit for slewing the upper slewing body 2, the pressure of the slewing motor exceeds the relief pressure, particularly at a starting time and an acceleration time of the slewing, to thereby increase a relief flow rate, that is, a flow rate of the hydraulic fluid let to the tank by the relief action, resulting in large relief loss.
Japanese Unexamined Patent Publication No. 2011-208790 discloses a relief cut control for suppressing a relief loss at the slewing starting time and the like. The relief cut control involves detecting a slewing speed, determining a target pump flow rate Qo, and adjusting a tilt angle of the hydraulic pump for obtaining the target pump flow rate Qo. The target pump flow rate Qo is the sum of a flow rate Q1 corresponding to the detected slewing speed (a flow rate of actual flow to the slewing motor; hereinafter, referred to as a “speed-correspondence flow rate”), and a “minimum required relief flow rate” Qmin which is a relief rate required for obtaining a minimum pressure required for starting slewing starting, the minimum pressure being a property value of the relief valve.
This conventional technique, however, takes no account of change in the pump flow rate involved by the change in the engine revolution number, though the engine revolution number varies depending on working and the like. The conventional technique, therefore, generates a risk of permitting the change in the engine revolution number to make the minimum required relief flow rate Qmin too small or too large. Specifically, setting for obtaining the minimum required relief flow rate Qmin with a relatively high idle engine speed involves a risk of shortage in the pump flow rate with the relatively low idle engine speed, which may prevent pressure required for slewing from being generated to thereby make it impossible to start or accelerate slewing. Reversely, setting for obtaining the minimum required relief flow rate Qmin with a relatively low idle engine speed generates a risk of making the pump flow rate too large with the relatively high idle engine speed, which prevents energy saving as an original object of the relief cut from being achieved.